Falsely normal value in fluorometric transferase screening of galactosemic blood. A cautionary note

A blood sample from a galactosemic infant gave a normal result with the fluorescent screening test for galactose-1-phosphate uridyl transferase. The generation of fluorescence from this sample was found to be property of the plasma; the erythrocytes manifested no galactose-1-phosphate uridyl transferase activity. The plasma was found to contain a high level of isocitrate dehydrogenase, presumably a result of the infant's liver disease, and of isocitrate, presumably derived from citrate in the anticoagulant solution by action of aconitase derived from leukocytes which lysed during shipment. The oxidation of the isocitrate by the dehydrogenase apparently resulted in reduction of NADP to NADPH. False negative results have not been reported previously using the fluorescent transferase assay. They can be avoided in the future by using EDTA rather than citrate as an anticoagulant, particularly if blood samples are to be shipped at ambient temperature.